The present invention relates to processing image data in which an image frame is divided into a plurality of pixel values.
Many procedures for modifying image data after the data has been recorded are known and collectively these procedures are often referred to as Post Production Compositing is a post production technique in which portions of a first image are combined artificially with portions of other images to produce a composite image.
When compositing of this type is to take place, the compositing procedures may be assisted by adopting appropriate techniques during the production process, such as recording foreground images against blue screen or green screen backgrounds etc. Under these circumstances, foreground images may be removed from a background screen using a process of chroma-keying.
A system in which each pixel is represented by three colour components defining a position within colour-space is disclosed in International patent publication WO 98/11510. A base colour is identified and a distance in colour-space between an input colour and said base colour is calculated. The control value, which may be used as a basis for a chroma suppress or a chroma-key etc, is produced in response to the calculated distance.
Chroma-keying techniques are being improved so as to extend the type of images where this technique may be used and also to increase the speed at which the chroma-keying process takes place. However, there are some situations where it is desirable to separate a first portion of an image from a second portion of an image, or at least to identify a boundary between the images, from source material where chroma-keying techniques would be ineffective. In particular, chroma-keying is not possible when the same or very similar colours are present in both the first portion of the image and the second portion of the image.
According to a first aspect of the present invention, there is provided a method of processing image data in which an image frame is divided by a plurality of pixel values, comprising steps of viewing an image frame in which a perceived object is to be distinguished from the remainder of the image frame; manually identifying positions in proximity to the boundary of said object; comparing pixel values around each identified position to define local colour transitions; and interpolating between said transitions to produce a boundary for the object.
Preferably, at each control point, a first region is identified at a position on a first side of the boundary; a second region is selected at a position on the second side of the boundary; and the colours of said selected regions are compared to identify an accurate boundary position. Preferably, the position of the first region and the position of the second region are identified by shapes connected together by a line crossing the boundary. Preferably, the length of said line is adjustable.
According to a second aspect of the present invention, there is provided image processing apparatus, comprising storage means for storing image frames as a plurality of pixel values; viewing means for viewing an image frame in which a perceived object is to be distinguished from the remainder of the image frame; input means configured to receive input signals identifying control positions in proximity to the boundary of said object; comparing means configured to compare pixel values around each identified control position to define a local colour transition; and interpolating means configured to interpolate between said colour transitions to produce a boundary for said object.